Optical communication systems based on coherent detection and digital signal processing (DSP) are known in the art. Coherent detection refers to a mode of fiber optic communication where the receiver exploits knowledge of the carrier's phase to detect and recover the underlying signal. Digital signal processors permit relatively low cost, highly customizable, and high speed mathematical manipulation of digital information signals. Coherent detection and digital signal processing have proven indispensable in ultra-high speed optical transport to improve receiver sensitivity and achieve superior channel equalization of signal impairments.
In optical fiber transmissions, polarization effects such as polarization mode dispersion (PMD) and nonlinear polarization coupling are known to degrade transmission performance. PMD is a form of dispersion where two different polarizations of light in a waveguide travel at different speeds due to random imperfections and asymmetries in the path, resulting in random spreading of optical pulses. Due to the physical nature of the polarization related effects, the magnitude of this performance degradation is dependent on the state of polarization (SOP). For example, the most significant signal distortion induced by PMD happens when the SOP of a transmitted signal has a 45 degree angle with respect to the two principle axes of an optical fiber channel. By contrast, a signal with an SOP that is aligned with one of the two principle axes can propagate through the optical fiber channel with no PMD induced distortion. Signals with SOPs that vary between these extremes suffer varying degrees of PMD induced distortion.
These polarization dependent impairments can be mitigated to some extent using techniques for polarization scanning and scrambling. A polarization scanner converts any input state of polarization to any desired state of output polarization. A polarization scrambler rapidly varies the polarization of light within a system using a polarization controller so that the polarization related effects are averaged over different SOPs.
In contemporary optical fiber communication systems, polarization scanning and scrambling may be implemented with electro-optic devices such as lithium niobate (LiNbO3) devices placed in the path of an optical signal. Polarization scramblers vary the normalized Stokes vector of the polarization state over the entire Poincaré sphere. The Stokes parameters are a set of values that describe the polarization state of electromagnetic radiation. The Poincaré sphere provides a model for visualizing the last three Stokes parameters in spherical coordinates. Lithium niobate devices are effective at mitigating polarization dependent effects but they are relatively expensive and require physical modification of the light path in an optical fiber transmission system and lack the flexibility of digital processors.